SUMMARY

Phenotypic plasticity describes the capacity of a genotype to exhibit a
range of phenotypes in response to variation in the environment. Environmental
variation encompasses both abiotic and biotic components of the environment,
including interactions among organisms. The strength and outcome of many
ecological interactions, ranging from antagonism to mutualism, are mediated
through the phenotypically plastic responses of one or more players in the
interaction. Herein, three broadly defined, non-mutually exclusive,
evolutionary consequences of ecological interactions mediated through
phenotypic plasticity are discussed. (1) The predictable plastic response of
one partner can favor behaviors, physiological responses, and life history
traits of an interacting partner that manipulate, circumvent, or ameliorate
the response of that partner. (2) Phenotypic plasticity can generate
substantial spatial and temporal variation within and among populations. Such
phenotypic variation can depend on the density and identity of interacting
players in an ecological community, and can ultimately affect the evolutionary
outcome of ecological interactions. (3) Phenotypic plasticity affects the
strength and direction of natural selection. Ecological interactions mediated
through phenotypic plasticity are ubiquitous in nature, and the potential
evolutionary consequences of these interactions illustrate the complexity
inherent in understanding evolution in a community context.

Introduction

Phenotypic plasticity refers to the ability of a genotype to exhibit
alternative morphological, behavioral and physiological characteristics in
response to environmental conditions
(West-Eberhard, 1989;
Garland, Jr and Kelly, 2006).
The ability of an organism to respond to its surroundings can be critical for
its success, especially in heterogeneous or novel environments, because it is
a means for an organism to explore the adaptive landscape
(Scheiner, 1993;
Via et al., 1995;
Price et al., 2003).
Plasticity may also play an important role in evolution by governing or
modifying developmental pathways
(West-Eberhard, 2003). Thus,
phenotypic plasticity plays a paramount role in generating the phenotypic
diversity observed in nature.

Plasticity can affect both the mean and variance of a phenotype within a
population (Fig. 1). A shift in
the mean phenotype of a population can occur when all individuals in a
population respond similarly to an environmental cue
(Fig. 1A). The variance
observed for a trait can similarly be reduced when a common response is
observed among members of a population
(Fig. 1B). Alternatively, the
variance can increase if individual genotypes within a population respond
differently to the same cue (Fig.
1C). The influence of plasticity on the mean and variance of a
population's phenotype will be influenced by the time scale over which
plasticity is expressed, genetic variation for plasticity in the population,
and heterogeneity of the environmental cues responsible for the plastic
response.

The time scale over which a plastic response is expressed can be almost
immediate, such as some physiological responses, behavioral responses of
animals (West-Eberhard, 2003),
and rapid movement of some plants
(Forterre et al., 2005).
Alternatively, plastic responses can be comparatively slow, such as
morphological alterations exhibited by animals in response to diet
(Wainwright et al., 1991;
Price, 2006) or by plants in
response to habitat (Ackerly et al.,
2000). Plastic responses can also vary in their permanency. Some
plastic responses, like behavior, are rapidly reversible, whereas other
responses are developmentally fixed, such as the discrete mimetic morphs of
Nemoria arizonaria (Geometridae) caterpillars
(Greene, 1989). Many plastic
responses fall between these two extremes and are at least partially
reversible. The reversibility of a plastic response can be dependent on the
ontogenetic stage when a response is induced. For example, the reversibility
of predator-induced morphologies exhibited by tadpoles decreases over
development (Relyea, 2003).
Induced phenotypes can be brief, or persist for years
(Karban and Baldwin, 1997;
Tollrian and Harvell, 1999),
and may even persist across generations
(Agrawal et al., 1999). The
temporal dynamics of phenotypic plasticity can also present individual
organisms with unique evolutionary challenges. For example, clouded sulphur
butterfly larvae (Colias philodice) can feed on a range of legumes at
early stages of development; however, they become obligate specialists on the
plant species they consume during the penultimate instar
(Karowe, 1989). Thus, over the
course of their life, they experience the evolutionary and ecological
trade-offs associated with a specialist and generalist diet
(Futuyma and Moreno, 1988;
Jaenike, 1990).

No organism is so insular that it does not have some interactions with
other organisms, whether these interactions occur between members of the same
or different species. The term `interaction' describes how one organism
affects another organism. The strength and direction of ecological
interactions are ultimately determined by ecological conditions, including
abiotic factors and the phenotypes of the participants. Interactions between
organisms range from antagonism, such as those between predators and prey, to
mutualism, such as those between plants and pollinators. The study of
ecological interactions is challenging because the effect of the interaction
on any given participant, whether it be positive, negative or neutral, often
depends on the current ecological conditions
(Thompson, 2005). The
direction of an interaction may be antagonistic under one set of ecological
conditions, yet neutral or beneficial under alternative conditions (e.g.
Masters and Brown, 1992;
Thompson, 1988;
Nykänen and Koricheva,
2004). For example, the presence of ant-tended herbivores can
actually be beneficial to a plant when the tending ants also defend the plant
against other herbivores (Messina,
1981). From the perspective of the plant, the direction of the
interaction depends on the presence or absence of other herbivore species that
can cause severe damage. This rule of conditionality extends to interactions
that are affected by phenotypic plasticity of one or more players in an
interaction.

Scenarios for change in mean and/or variance of a trait in a population
between the constitutive phenotype expressed prior to an interaction (blue)
and the induced phenotype following an interaction (red). (A) An increase in
mean and variance of a trait. (B) Decrease in variance, mean unchanged. (C)
Increase in variance, mean unchanged.

The role of environmental conditions affecting phenotypic variation in a
population has long been recognized in basic additive genetic models, where
phenotypic variation is partitioned between genetic effects and environmental
effects. The contribution of environmental effects to phenotypic variation
represents overall plasticity. The interaction between genotype and
environment represents genetic variation for phenotypic plasticity. Although
phenotypic responses to the environment may not increase reproductive success
(i.e. fitness) and are often referred to as environmental noise, they still
can be considered plasticity because alternate phenotypes are produced by the
same genotype (Connor and Hartl,
2004; Garland, Jr and Kelly,
2006). Such non-adaptive, at times maladaptive, phenotypic
responses can influence the strength and direction of ecological interactions
and, therefore, can affect the fitness of one or more interacting partners.
Adaptive plasticity describes phenotypic responses to the environment that
results in increased fitness for an organism, such as inducible defenses of
plants in response to herbivore attack
(Karban and Agrawal,
2002).

Environmentally influenced phenotypic variation can be further partitioned
between abiotic and biotic components of the environment. Here, the
interaction term between abiotic and biotic environmental factors describes
how plastic responses to an interaction can vary over different conditions.
For example, the capacity of plants to respond to herbivore attack can be
affected by nutrient availability (Lou and
Baldwin, 2004). Biotic components of the environment include
interactions with other organisms and will largely be the focus of the current
discussion.

Many ecological interactions are mediated through the phenotypic plasticity
of participants in response to other participants
(Karban and Baldwin, 1997;
Tollrian and Harvell, 1999;
Agrawal, 2001;
Werner and Peacor, 2003;
Callaway et al., 2003;
Miner et al., 2005). The
strength and direction, and ultimately the outcome, of these interactions will
be influenced by ecological conditions, including the genotypes involved in an
interaction and the capacity for reciprocal phenotypic responses among the
participants (Agrawal, 2001).
Many research programs have been dedicated to understanding plastic responses
and ecological interactions, often in the context of induced resistance or
defense observed in antagonistic interactions, such as immune responses to
pathogens (Dangl and Jones,
2001), predator-induced morphology and behavior (Tollrian and
Harvell, 1997; Preisser et al.,
2005), and herbivore-induced plant responses
(Karban and Baldwin, 1997).
Mutualistic interactions can also be mediated through plasticity, for example
by modifying the quantity or quality of rewards provided by one partner in the
interaction (Leimar and Axen,
1993).

Plasticity can also be an important agent for indirect interactions among
members in an ecological community. For example, damage caused by an herbivore
can induce a plant phenotype that affects the performance of other herbivores
(Masters and Brown, 1992;
Denno et al., 1995;
Viswanathan et al., 2005;
Fordyce, 2006). Although many
examples of phenotypic plasticity exhibited in response to ecological
interactions have been shown to be beneficial, such as induced resistance or
defense, this is not always the case. For example, some herbivore-induced
plant responses can increase a plant's susceptibility to further attack
(Underwood, 1998). Thus,
spatially and temporally separated herbivores can have indirect interactions
with each other mediated through the phenotypically plastic responses of the
plant.

Phenotypic plasticity plays a substantial role in generating variation in
the strength and direction of ecological interactions. In this review, I focus
on the evolutionary consequences of interactions among species that are
mediated through phenotypic plasticity. These consequences are separated into
three, non-mutually exclusive, facets of interactions mediated by plasticity;
(1) predictability of plastic responses, (2) spatial and temporal variation
caused by plasticity, and (3) plasticity's effect on the fitness landscape and
natural selection.

Predictability

Predictable plastic responses are those that are reliably expressed across
environments following an interaction. Predictable plastic responses provide
the opportunity for the evolution of strategies that ameliorate, avoid or
manipulate the anticipated phenotypic response of an interacting partner.
These strategies include behaviors, physiological responses and life-history
evolution.

Many herbivores that feed on plants possessing laticifer systems disrupt
latex flow prior to feeding by severing the latex bearing canals
(Dussourd, 1993;
McCloud et al., 1995). The
ability of herbivores to anticipate latex exudation and manipulate the
effectiveness of this plant response to damage can substantially influence
herbivore diet breadth (Dussourd and
Denno, 1994). Recently, it was shown that larvae of the corn
earworm (Helicoverpa zea) adjust their physiology in anticipation of
induced plant responses by `evesdropping' on the plant's physiology
(Li et al., 2002). The larvae
altered the quantity and quality of gut P-450 enzymes in response to plant
salicylates and jasmonates, signaling compounds in plants associated with the
induction of defensive compounds. As a consequence, the larval gut was
prepared prior to the induction of plant defensive compounds, thereby reducing
the efficacy of the induced plant response.

The phenotypically plastic responses of a participant in an interaction can
also be manipulated through life history traits. For example, large clutch
size and aggregative feeding of larvae of the pipevine swallowtail butterfly
(Battus philenor) can be an effective strategy for increasing host
plant suitability, thereby accelerating larval developmental rate
(Fordyce, 2003). The
effectiveness of group feeding as a manipulative strategy varies among
potential host plant species. Populations using host plants that cannot be
manipulated through aggregative feeding usually lay substantially smaller
clutches and larvae do not feed in dense aggregations
(Fordyce and Nice, 2004). The
ability of larvae to manipulate host plant quality through aggregative feeding
might explain geographic variation in clutch size.

Many parasites manipulate the behavior of their hosts in ways that enhance
parasite transmission and survival (e.g.
Moore, 1984;
Stamp, 1981;
Lafferty, 1999), and for some
parasites such behavioral manipulation is necessary to complete their life
cycle. For example, Thomas et al. showed that a parasitic nematomorph induced
a behavior in their insect host that caused them to enter the aquatic habitats
required for the parasite's life cycle, even though the behavior is lethal for
their host (Thomas et al.,
2002). Various species of rust fungi manipulate the phenotype of
their host plant, inducing the formation of structures called psuedoflowers
that mimic the appearance of flowers. These floral mimics are visited by
pollinators, which disperse fungal spores and facilitate cross-fertilization
of mating types (Roy, 1993;
Pfunder and Roy, 2000). Such
strategies demonstrate that manipulation of plastic responses can affect both
direct interactions between participants and indirect interactions among
community members.

The ability to manipulate the phenotype of an interacting partner can
provide an opportunity for entry into a novel adaptive zone, facilitating
diversification of a lineage (i.e. `key innovation')
(Simpson, 1953). For example,
the ability to form plant galls, structures formed by abnormal growth of plant
tissues, has arisen in at least seven orders of insects and has led to an
extraordinary adaptive radiation of some groups, such as gall wasps
(Cynipidae) and gall midges (Cecidomyiidae)
(Weis et al., 1988;
Inbar et al., 2004). Gall
formation is such an intrinsic quality of some insect life histories that the
gall itself, an expression of the plant's genotype, can be regarded as the
extended phenotype of the gall former
(Dawkins, 1982). The ability
to effectively manipulate the host phenotype, usually in very specific ways,
restricts the host plant breadth of gall formers. As a consequence of this
intimate association, nearly all gall-forming insects are extremely
specialized for particular host plant species.

Spatial and temporal variation

The strength and direction of ecological interactions vary in time and
space. Genetic variation within and among populations can be an important
determinant of phenotypic variation, which in turn can affect ecosystem
processes (Madritch and Hunter,
2002; Schweitzer et al.,
2004) and play an important role in determining the composition
and organization of ecological communities
(Antonovics, 1992). For
example, increased genetic variation in plant communities can lead to
increased richness of arthropod communities
(Wimp et al., 2004;
Johnson et al., 2006). A key
component of community genetics is the emphasis on the role of genetic
variation affecting phenotypic variation within a community (Whitham, 2003).
Phenotypic plasticity in response to ecological interactions can similarly
play an important role in generating phenotypic variation within and among
populations (Karban and Baldwin,
1997; Tollrian and Harvell,
1999; Roff, 1992;
Agrawal, 2005).

Plasticity in response to ecological interactions can affect the mean of a
population phenotype (Fig. 1).
Predator threat can induce population-wide behavioral and morphological
responses of their prey. For example, many amphibians and aquatic insect
larvae adjust their behavior and morphology in response to predator threat
(Benard, 2004).
Predator-induced responses can persist in populations and extend beyond the
community where they were induced. Wood frog tadpoles (Rana
sylvatica) not only change morphology in the presence of predators in
their aquatic environment, but emerging metamorphs have relatively long fore-
and hindlimbs (Relyea, 2001).
The chemical defenses of the western toad (Bufo boreas) metamorphs
were higher when tadpoles were reared in the presence of predator cues
(Benard and Fordyce, 2003). As
a consequence of phenotypic plasticity, the mean phenotype of populations
occurring in high predator environments might differ substantially from
populations where predators are absent
(Abrams, 1984;
Tollrian and Harvell, 1999;
Werner and Peacor, 2003;
Preisser et al., 2005). For
example, predator cues increased the level of tadpole aggregation and led to
more synchronous metamorphosis of the American toad (B. americanus)
(DeVito, 2003). Similarly,
herbivore-induced responses can change the mean phenotype of a plant
population, such as changes in nutritional quality, chemical defenses and
structural defenses (Karban and Baldwin,
1997).

The variance of a trait can be reduced in a population in response to
biotic interactions. Behavioral plasticity in response to predator threat can
markedly reduce the size of a prey population's realized niche, reducing the
range of occupied habitats and possibly the amount of available resources
(Preisser et al., 2005). Other
population responses to biotic interactions, such as premature leaf abscission
of plants in response to herbivory
(Simberloff and Stiling, 1987)
and accelerated maturation or dormancy in response to predators
(Lass and Spaak, 2003), also
have the potential to greatly affect the variance of traits relevant to an
interaction. An increase in phenotypic variance due to plasticity can result
if individuals within a population respond idiosyncratically to an interaction
or if genetically based variation in plasticity is present in the population.
Idiosyncratic responses to an interaction, by definition, are not predictable.
Thus, interacting partners must be able to cope with a wide range of
phenotypes and the evolution of manipulative strategies will be less likely,
unless these strategies circumvent or prevent an induced response.

The mean and variance of phenotypic traits relevant to interactions can
also be affected by the diversity of participants in the interactions. For
example, prey may respond to predator threat by inducing behaviors or
morphologies specific to a particular predator
(Sih et al., 1998;
Kishida and Nishimura, 2005).
Similarly, the herbivore-induced phenotypes of plants can vary according to
the identity of the herbivore. Herbivore-induced responses can range from
increased resistance against further herbivory to increased vulnerability to
subsequent attack, and can be specific to particular herbivore species. The
identity of herbivores inflicting damage on the common milkweed (Asclepias
syriaca) early in the season influenced which species of herbivores
colonized the host plant later in the season
(Van Zandt and Agrawal, 2004).
Thus, plasticity can generate an important feedback between induced phenotypes
and community composition. The assembly of an herbivore community on a plant
can depend on the identity or density of particular community members that
affect plant phenotype. Such community architects have been termed `keystone
herbivores' (Hunter, 1992;
Gonzáles-Megias and Gómez,
2003); analogous to Paine's `keystone predator' concept
(Paine, 1966). Removing a
keystone herbivore results in an alternate plant phenotype, affecting the
distribution and abundance of other community members, including herbivores
and herbivore natural enemies.

Community composition, and thus phenotypic variation caused by plastic
responses to interactions, varies in space. Within-population variation can be
altered if there are multiple players involved in an interaction.
Among-community geographical variation in the presence of important
participants in an interaction, such as keystone herbivores, can increase
among-population differences. Thus, plasticity in response to ecological
interactions can generate patterns of phenotypic variation similar to that
caused by genetic variation. Among-community variation in the strength and
direction of interactions can profoundly influence geographic variation in
evolutionary trajectories (Thompson,
1999; Thompson,
2005; Ridenhour,
2005).

The regularity with which particular ecological interactions occur varies
among communities. Thus, some populations might consistently be involved in a
particular interaction, such as competitive interactions between plant
populations in the presence of a keystone herbivore, whereas other populations
may not. Among-population phenotypic variation can be a consequence of the
plastic responses unique to local conditions; however, these differences can
eventually lead to genetically based differences
(Pigliucci and Murren, 2003).
For example, among-population variation was found in the propensity of spade
foot toad tadpoles (Spea multiplicata) to exhibit a carnivorous
phenotype, a plastic response to the presence of particular prey items
(Pfennig and Murphy, 2002).
The carnivorous phenotype was more commonly expressed in populations that were
not sympatric with the superior competitor, S. bombifrons.

The evolutionary outcome of biotic interactions, whether it is the response
of one participant in an interaction or coevolution, will be influenced by the
phenotypic variation present for traits relevant to the interaction. If the
population of one participant involved in an interaction has little phenotypic
variation, all individuals of the interacting partner will be under similar
selection pressure, facilitating adaptive evolution in the population.
However, adaptive evolution in response to an interacting partner is more
difficult when individuals within a population interact with different
phenotypes. Phenotypic plasticity can play an important role in slowing the
adaptive response of an interacting partner by providing a `moving target' for
natural selection (Adler and Karban,
1994). This basic principle has been used to retard the
evolutionary response of insect pests and pathogens to genetically modified
crops or pesticides (Rausher,
2001).

Natural selection, phenotypic plasticity and ecological
interactions

Plasticity can affect the strength and direction of natural selection that
occurs between interacting species. Natural selection affects the frequency
distribution of phenotypes in a population. How a population will respond to
selection can be graphically illustrated by plotting the phenotypic
distribution of a trait against mean fitness (assuming no frequency
dependence) (Lande, 1979;
Lande and Arnold, 1983;
Brodie et al., 1995;
Price, 2006). If the adaptive
surface remains unchanged, populations will evolve a mean phenotype that
maximizes mean fitness. However, phenotypic plasticity of an interacting
partner can result in dynamic adaptive surfaces that change over time in
response to interactions (Fig.
2). In short, the fitness surface of one participant in an
interaction can be determined by the plastic response of its partner.

Plastic responses of one participant might favor a wider range of tolerance
for that trait in its interacting partner. For example, herbivore-induced
changes in plant chemistry might favor herbivore phenotypes that have
tolerance or the ability to cope with a broad spectrum of host plant chemistry
(Fordyce, 2001). Similarly,
plasticity of one partner might select for the capacity of the other partner
to express counter plastic responses
(Broadway, 1996;
Agrawal, 2001;
Kopp and Tollrian, 2003). The
ability for reciprocal phenotypic plasticity will be important if the fitness
of the constitutive phenotype is greatly reduced as consequence of plasticity
in the interacting partner. Thus, plasticity expressed by one community member
creates a variable biotic environment that might favor the evolution of
plasticity in other community members.

A scenario where the plastic response of one partner affects the fitness
surface of an interacting partner; here an example between an insect herbivore
population and its host plant. The red curves indicate the phenotypic
distribution (middle) and fitness surface (top) of the insect herbivore. The
green curve indicates the distribution (bottom) of host plant traits relevant
to the interaction. (A) Prior to the plastic response of the plant, the traits
of the herbivore mirror those of the plant. If the phenotypic distributions
remain constant, stabilizing selection will occur, selecting against extreme
phenotypes with lower relative fitness. (B) A plastic response of the plants
shifts the distribution of phenotypes in the population. As a consequence, the
fitness surface of the herbivore changes and directional selection occurs.
Note that it is possible for the herbivore to have a complementary plastic
response, which will change its phenotypic distribution and the shape of the
fitness surface.

The time scale over which plastic phenotypes are expressed will undoubtedly
influence the evolutionary response of the interacting population. Whereas
plasticity that plays an important role within a generation might favor
broader tolerance or reciprocal plasticity, plastic responses in one species
that extends over multiple generations of the other might lead to shifting
selective pressures across generations. For example, changes in foliar
chemistry of trees following herbivore outbreaks can persist for years,
potentially spanning several insect generations
(Haukioja, 1990;
Karban and Baldwin, 1997).
Thus, many insect herbivores experience periods of selection imposed by
increased defensive chemistry, followed by a period of relaxed selection. Such
temporal, across-generation, variation in the strength and direction of
selection might be important for maintaining genetic variation in
populations.

Plasticity in response to interactions can alter the relative contribution
of genetic variation to phenotypic variation, and ultimately the strength and
direction of selection. From an evolutionary perspective, this reduces the
likelihood of strict, trait-for-trait pairwise coevolution between interacting
partners, illustrating why diffuse coevolution is assumed to occur commonly in
antagonistic interactions, such as those between plants and natural enemies
(Rausher, 1996;
Thompson, 1999;
Inouye and Stinchcombe, 2001;
Strauss and Irwin, 2004).
Plasticity can affect a population's ability to respond to selection imposed
by particular community members because the phenotypes expressed are
conditional on prior interactions. Take a hypothetical example where early in
a season the variation observed in a population for a trait relevant to an
interaction is largely explained by genetic variation. The proportion of
phenotypic variation that is explained by genetic variation describes the
heritability of the trait, or the ability of the trait to respond to
selection. Over the course of the season, the phenotypic variation present in
the population is increasingly conditional on interactions with other
community members accumulated over time. Thus, a consequence of plasticity can
be temporal variation in the heritability of traits relevant to an
interaction. Pilson observed genetic variation in goldenrod (Solidago
altissima) for resistance to various herbivores that damage the apical
meristem causing the plant to branch
(Pilson, 1992). When
branch-causing herbivores were present, the plant genotype predicted the
distribution of aphids because aphids were more likely to be found on plants
with multiple branches. However, when branch-causing herbivores were absent,
aphids were randomly distributed among genotypes. Thus, the appearance of
genetic variation for resistance to aphids was contingent on previous
interactions with branch-causing herbivores.

Conclusions

The diverse interactions occurring in communities coupled with the
conditional phenotypic responses of participants show that phenotypic
plasticity can have profound evolutionary consequences, both within and among
populations. When a plastic response of one interacting partner is dependably
expressed, life history and behavioral strategies can evolve in anticipation
of these predictable responses. The prevalence of such anticipatory
strategies, including those that manipulate the phenotype of an interacting
partner, remains to be seen. Phenotypic plasticity can generate patterns of
variation similar to those resulting from genetic variation. What is unknown,
however, is the relative importance of genetic diversity versus
phenotypic plasticity in generating variation that influences the composition
of ecological communities. Understanding the adaptive evolutionary responses
of interacting partners requires an appreciation that plasticity determines
the interacting phenotypes, and ultimately can mediate shifting selective
pressures.

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Are you going to the 2018 APS Comparative Physiology meeting in October? This meeting is supported by JEB and our Reviews Editor Charlotte Rutledge will be there - stop by and say hello! Advance registration is open until 24 September 2018.